1. Field of the Invention
The present invention relates to a thermal head for feeding an electric current corresponding to a print signal to plural heating elements selectively to heat, and printing on a recording medium such as thermal sensitive paper and heat transfer film, preferably used in facsimile apparatus, image recording apparatus or the like, and a method for driving the same.
2. Description of the Related Art
FIG. 8 is a block diagram of a first example of a conventional thermal head 1. In the thermal head 1, heating resistive element 4 are connected to a common electrode 2, and switching elements 6 such as power transistors are individually connected to the other ends of the heating resistive element 4 through individual electrodes 5. The output terminals of the switching elements 6 are commonly connected to a grounding wire 7, and AND elements 8 are connected to the control signal input terminals of the switching elements 6. The switching elements 6 and AND elements 8 are formed in a driving circuit element 9 formed by integrated circuit technology, and also in the driving circuit element 9, a shift register 10 in the same number of bits as the number of all AND elements 8 and a latch circuit 11 are formed.
In this related art, assuming there are twenty-four heating resistive element 4, they are divided into four blocks B1 to B4 of six elements each in the sequence of array. Therefore, twenty-four pieces each are used for the switching elements 6 and AND elements 8, and strobe signals XSB1 to XSB4 are fed from a control device 13 to the AND elements 8 in every one of the blocks B1 to B4. The strobe signals XSB1 to XSB4 are low-active signals, and inverters 12 are provided in every one of strobe signals XSB1 to XSB4.
FIG. 9 is a timing chart for explaining the operation of the thermal head 1 in FIG. 8. In the shift register 10, print data D is fed as serial signals together with clock signal CK as shown in FIG. 9 (1) and FIG. 9 (2). At a predetermined timing, a latch signal LT in FIG. 9 (3) is fed into a latch circuit 11, and the data stored in the shift register 10 is latched. Afterwards, strobe signal XSB1 in FIG. 9 (4) is commonly fed to each AND element 8 of the first block B1.
Consequently, the print data D latched in the latch circuit 11 is sent out to the switching elements 6 of the first block B1. The switching elements 6 are set in the conductive or cut-off state depending on the print data, and in the conductive state the current from the common electrode 2 flows through the heating resistive element 4 into the grounding wire 7, and this heating resistive element 4 is heated and driven. In sequence, the corresponding strobe signals XSB2 to XSB4 are sequentially applied to the blocks B2 to B4, and the printing action continues.
While the strobe signal XSB4 is being produced, the print data D of the next line is sent out from the control device 13 together with the clock signal CK, and stored in the shift register 10. The storing timing of the print data D of the next line to the shift register 10 may be any period of output of the strobe signals XSB1 to XSB4 as far as after the output of the latch signal LT.
In the conventional thermal head 1 in FIG. 8, however, in the driving circuit element 9, the AND elements 8 are formed in the same number as the heating resistive elements 4, and the shift register 10 and latch circuit 11 require the same number of bits as the number of the heating resistive element 4, and therefore the constitution of the driving circuit element 9 is complicated, and the cost also increases.
Besides, as the constitution is complicated, the driving circuit element 9 becomes larger in size, and it is difficult to downsize the thermal head 1.
Moreover, since the print action period and data transfer period are set separately, the entire printing time becomes longer.
FIG. 10 is a timing chart showing the circuit action in the constitution, omitting the latch circuit 11 for simplifying the constitution in the thermal head 1 in FIG. 8, in which the data from the shift register 10 is fed in the corresponding AND elements 8 in every bit. In such structural example, since the latch circuit 11 is not present, in the period when the strobe signals XSB1 to XSB4 are sent to the data from the shift register 10, new print data cannot be stored in the shift register 10. Accordingly, As shown in FIG. 10 (1) to (6), after the print data D of one line portion is stored in the shift register 10 together with the clock signal CK, the strobe signals XSB1 to XSB4 are produced in time sequence, and after print action of one line is over, the print data of the next line is stored in the shift register 10.
In such conventional example, therefore, the print action time becomes longer.
FIG. 11 and FIG. 12 are timing charts showing other examples of the print action of the thermal head 1 in FIG. 8. In the thermal head 1, in order to heat the heating resistive element 4 to thermally record on a thermal sensitive recording paper or the like, when the thermal head 1 continues the printing action or when the environment of use is relatively high in temperature, the current passing time of the heating resistive element 4 per predetermined density is set shorter as compared with the case when beginning to use the thermal head 1 or when the environment of use is relatively low in temperature. That is, the thermal head 1 in FIG. 11 corresponds to the current passing time when relatively high in temperature, that is, to the pulse width T1 of the strobe signals XSB1 to XSB4, while FIG. 12 refers to the case in which the thermal head 1 is relatively low in temperature. That is, the pulse width T2 of the strobe signals XSB1 to XSB4 at low temperature is set longer than the pulse width T1 in FIG. 12.
Such pulse width is determined so that the total of the pulse widths of four strobe signals XSB1 to XSB4 may equal the print time of one line, or periodic interval T3 of latch signal LT when the thermal head 1 is at the predetermined lowest temperature as shown in FIG. 12, and the pulse width gradually decreases as the temperature of the thermal head 1 rises.
That is, in the thermal head 1 controlled by such control method, when the pulse width of the strobe signals XSB1 to XSB4 is maximum, it is necessary to control so as to store the print data of the next line until the output of the final strobe signal XSB4 is over in one line, and it is difficult to set the print period of the strobe signals XSB1 to XSB4 and the store period of print data in different time ranges as explained in FIG. 10.
FIG. 13 is a block diagram of a second example of a conventional thermal head la for solving the above problems. In this related art, the parts corresponding to those in the first conventional example are identified with same reference numerals. In this example, the latch circuit 11 is omitted in the constitution in FIG. 8, and four shift registers 10a to 10d are provided corresponding to four blocks B1 to B4 of the heating resistive element 4, print data D1 and clock signal CK1 are fed in the shift register 10a from a control device 13, and similarly thereafter, print data D4 and clock signal CK4 are fed in the shift register 10d from the control device 13.
The print action of this thermal head la is shown in the timing chart in FIG. 14. That is, as shown in FIG. 14 (1) and (2), the print data D1 is fed in the shift register 10a together with the clock signal CK1 from the control device 13. After storing of data is over, the strobe signal XSB1 in FIG. 14 (9) is sent out, and the heating resistive element 4 of the block B1 are selectively heated to make a print. During the output period of the strobe signal XSB1, the control device 13 feeds the print data D2 of the block B2 into the shift register 10b together with the clock signal CK2 as shown in FIG. 14 (3) and (4). Thereafter, similarly, during the print action of the block Bi (i=1 to 4), the print data of the next block Bi+1 is stored in the corresponding shift register 10. During the print action period of the block B4, the print data of the block B1 of the next line is stored in the shift register 10a as shown in FIG. 14 (1) and (2).
In such example, in the conventional thermal head without latch circuit 11, although the printing speed is improved as compared with the case of printing action explained in FIG. 10, it is necessary to divide all heating resistive element 4 into four blocks, dispose shift registers 10a to 10d in each block, and feed print data and clock signal individually in each shift register 10a to 10d, Accordingly, the number of input and output terminals in the control device 13 and driving circuit element 9 increases, and the connection wirings for connecting these input and output terminals increase in number, so that the constitution of the thermal head la is complicated.